Various kinds of dot printers are known in the art. Early so-called "dot matrix" printers employed one or more pins driven forward and backward by a solenoid drive mechanism to transfer ink from a ribbon to the surface of a media as a series of dots or "pixels". This type of printer can be contrasted to laser printers, electrostatic printers, and the like, which form an entire two dimensional area as dots from a "toner" material which is then transferred to the surface of the media and fused to the surface by the application of heat. While mechanical dot matrix printers are still used in some applications, modern dot printers are more likely to employ a thermal inline printhead, a thermal printhead or an inkjet printhead.
Typically, a thermal printhead includes a plurality of print positions arranged in either or vertical or horizontal line. Each print position has a heating element connected to wires. When power is applied to the wires, the heating element increases in temperature. At a certain temperature, the heating element causes a visible dot to appear on the media being printed when employing thermal direct techniques where the heat is applied to a heat-sensitive coating on the surface of the media, or, where ink is transferred from a thermally sensitive ribbon to form a dot on the surface of the media.
The size and shape of the dot is a function of the shape of the heating element, power level, temperature of the heating element and the length of time the element is applied to the medium or ribbon. As the heating elements retain heat from previous printing operations and adjacent elements, the temperature of the heating elements rise. Thus, given a constant power level and velocity across a print medium, the size of the dots expands due to retained heat in the print heating element.
When using a conventional thermal printer, the heating elements are selectively heated to form a character as the thermal printing head travels in the printing direction at a predetermined pitch. As such, one character is formed by the dots for each character each time the thermal printing head is traveled by a predetermined number of dots. Once an entire row of characters is printed, the paper advances so that another row of information can be printed as the process is repeated. As speed of paper advancement increases, the amount of time available for the printhead to heat up and cool down decreases. The surfaces of these heating elements are heated up too much because of the retained heat from previous printing operations. This retained heat causes the printed characters to have variable density and sometimes characters have a horizontal trailing edge.
As electronically controlled printers continue to develop, in-line thermal printer heads have become popular. An in-line thermal printer head is a stationary head that uses a series of dot printing elements configured in a horizontal line across the width of the paper. As such, the head remains stationary with respect to the paper. The number of dot printing elements is a function of the print quality and the width of the paper. As opposed to a dot matrix printer, which prints a single character then moves a predetermined amount before printing another character, an in-line printhead selectively prints a horizontal row of dots across the paper at once. After a row of dots is printed, a drive system comprising a stepper motor and a system of gears and rollers, moves the media a predetermined distance along a paper path such that another row of dots can be printed. This process is repeated until the entire row of characters is printed on the media. Once the row of characters is printed, the paper is advanced so that another row of information can be printed as the process is repeated. As speed of paper advancement increases, the amount of time available for the printhead to heat up and cool down decreases. The surfaces of these heating elements are heated up too much because of the retained heat from previous printing operations. In an in-line printer, this retained heat causes the vertical elements of the printed characters to have vertical tails. For example, at slow printer speeds, the characters "J", "P", "I", "c", and "g" are printed according to a font as shown in FIG. 4. However, as these characters are printed at faster speeds, the result is illustrated in FIG. 5. The resulting characters have a remarkably lower character quality at higher print speeds.
Previous solutions have addressed this problem by building controllers and sensors into the printheads. These controllers control the pulsewidth of the printhead based on a combination of the temperature of the head measured by a sensor, the number of previous dots, and the number of adjacent dots printed. The pulsewidth is the amount of energy delivered to a print element. Thus, when the print element is already "hot," the pulsewidth can be reduced. This reduction in pulsewidth results in more uniform heating of the printheads and greater character quality. However, this solution requires the addition of controllers and sensors, resulting in higher printhead costs. Additionally, the printhead is physically larger necessitating larger and more expensive printhead support structure.
Past solutions have also included the use of a thermal print element history algorithm in the printer microcode. Such a solution also controls the pulsewidth by reducing the pulsewidth based on the previous history of the print element used in the printing process and the history of adjacent print elements. This solution requires extra processing time by the printer microprocessor resulting in higher CPU costs. Additionally, this solution requires significant time to initially develop and test the microcode.
What is needed, therefore, is a device or a method which economically and simply accounts for the distortion of characters due to the insufficient heating and cooling of the vertical printhead elements.